1. Field of the Invention
The present invention relates to a reference voltage generation circuit, and more particularly, to a reference voltage generation circuit that generates gamma voltages for liquid crystal displays.
2. Description of the Prior Art
With the widespread of cellular phones and personal data assistants (PDAs), different types of small-sized displays are required for such portable electronic devices. Among them, liquid crystal displays (LCDs) are the top choices for such small-sized devices due to low power consumption and high quality image display.
Generally, an image signal for displaying an image is subjected to gamma correction in accordance with a display characteristic of a display device. Gamma is a measure of contrast in an image and the way the brightness of an image is interpreted by certain hardware. In LCD devices, the gamma correction is carried out by a reference voltage generation circuit, namely the gamma correction circuit, in which multi-valued voltages are created in accordance to the transmittance of a pixel based on gray scale data for carrying out gray scale display. The gamma correction circuit generally comprises a resistance string having a plurality of serially arranged resistors and the voltages across two opposed ends of respective resistor circuits constituting the ladder resistor are outputted as multi-valued reference voltages in accordance with gray scale value.
FIG. 1 is a block diagram of an LCD device that includes a reference voltage generating circuit 100, an LCD panel 11, a gate driver 12, a source driver 13, and a timing controller 14. In the LCD panel 11, a plurality of gate lines are arranged to cross a plurality of data lines. The gate driver 12 sequentially applies a driving signal to the gate lines. The source driver 13 applies a data signal to the data lines. The reference voltage generating circuit 100 applies a reference voltage to the source driver 13. The timing controller 14 applies various control signals and voltages to the gate driver 12 and the source driver 13. The source driver 13 includes a shift register 15, a latch 16, a digital-to-analog converter (DAC) 18, and an amplifier 19. The source driver 18 firstly receives input digital data without processing. Then the DAC 18, controlled by the timing controller 14, converts the digital data to analog signals that can be applied to the LCD panel 11, and outputs the resultant values to each data line. At this time, the gamma voltages obtained by voltage division through resistors are outputted from the reference voltage generating circuit 100 to the source driver 13. The gamma voltages are varied depending on the LCD module.
FIG. 2 shows a prior reference voltage generating circuit 200 for generating gamma voltages disclosed in U.S. Pat. No. 6,731,259 to Jung Taeck Yer, Sin Hi Kang and Jong Dae Kim, which is included herein by reference. The reference voltage generating circuit 200 includes two voltage strings 23 and 25 arranged in parallel between a power source voltage terminal Vdd and a ground voltage terminal Vss, and an amplifier portion 27. The respective voltage strings 23 and 25 include a plurality of resistors R1 through R6 and R7 through R12 serially connected to generate a plurality of gamma voltages V1 through V10 through voltage division by the respective resistors. A corresponding amplifier of the amplifier portion 37 amplifies the gamma voltages V1 through V10 generated by the respective voltage strings 23 and 25. If the power source voltage Vdd is input, the gamma voltages from V1 to V10 are set by serially connected resistance values. At this time, voltages of a positive frame are set as the gamma voltages from V1 to V5 while voltages of a negative frame are set as the gamma voltages from V6 to V10. The reference voltage generating circuit 200 has a simple circuit structure and can be applied to most DACs. However, the reference voltage generating circuit 200 occupies a lot of space.
FIG. 3 shows another prior reference voltage generating circuit 300 for generating gamma voltages disclosed in U.S. Patent Publication. No. 2003/0151577 to Morita, which is included herein by reference. The reference voltage generation circuit 300 includes a positive polarity ladder resistor circuit 310 and a negative polarity ladder resistor circuit 320. The positive polarity ladder resistor circuit 310 generates reference voltages V1 to Vi used at a positive polarity inversion period through voltage division by a resistor string 312, and the negative ladder resistor circuit 320 generates reference voltage V1 to Vi used in a negative polarity inversion period through voltage division by a resistor string 322. In the reference voltage generating circuit 300, the positive polarity ladder resistor circuit 310 is coupled between a first power source line and a second power source line through switches SW1 and SW2 respectively, and the negative polarity ladder resistor circuit 320 is coupled between the first power source line and the second power source line through switches SW3 and SW4 respectively. The first power source line and the second power source line are set to fixed potentials Vdd and Vss respectively. Designated as S are switches controlled by a polarity inversion circuit. Designated as R are resistors of the resistor strings 312 and 322. Designated as VND1 through VNDi are reference output nodes of the reference voltage generating circuit 300.
In an LCD, the magnitude of the applied voltage determines the intensity of light emitted by pixel cells. To prevent polarization and rapid permanent damage of the liquid crystal material, the polarity of the cell voltage is reversed on alternative intervals. Therefore, in the case of line inversion and voltages of pixel cells of the same line have the same polarity, voltages of pixel cells of adjacent lines have opposite polarities against the upper common electrode; in the case of dot inversion, voltages of adjacent pixel cells of the same line have opposite polarities. Since the liquid crystal is made of a dielectric in this instance, charging and discharging of the liquid crystal will consume power during voltage polarity alternations. In the reference voltage generating circuit 300, the first power source line and the second power source line are set to fixed potentials Vdd and Vss, and a predetermined input voltage is required to establish a potential difference required for a positive polarity inversion period and a negative polarity inversion period. Since the first power source line and the second power source line are set to fixed potentials, the predetermined input voltage can not be changed during the positive polarity inversion period and the negative polarity inversion period. Therefore the reference voltage generating circuit 300 is quite power-consuming.